1. Field of the Invention
The present invention relates to an optical pickup apparatus, and in particular to an improved optical pickup apparatus which is capable of reading data from and writing the same onto discs each having a different recording density and thickness by selectively varying the numerical aperture of an objective lens using a liquid crystal shutter (LCS) and a diffraction hologram.
2. Description of the Conventional Art
FIG. 1 is a view illustrating the construction of a conventional optical pickup apparatus.
As shown in FIG. 1, a diffraction grating 2 is arranged beside a laser diode 1, which generates a laser beam, for dividing the beam from the laser diode 1 into a main beam and two sub-beams for a tracking servo.
In addition, an objective lens 4 is arranged beside a collimator lens 3 for focusing the parallel light from the collimator lens 3 on an optical disc D.
A beam splitting prism 5 is arranged between the diffraction grating 2 and the collimator lens 4 for transmitting the incident beam from the diffraction grating 2 at a predetermined ratio and for reflecting the beam reflected in accordance with the information written on the optical disc D.
A sensor lens 6 is arranged below the beam splitting prism 5 for condensing the beam reflected by the beam splitting prism 5, and an optical detector 7 is arranged for detecting a data signal of the beam passed through the sensor lens 6.
With the above-described construction, the beam from the laser diode 1 passes through the diffraction grating 2 and the beam splitting prism 5. The beam is then converted into a parallel light by the collimator lens 3 and focused by the objective lens 4. Thereafter, the beam is reflected or diffracted in accordance with the data recorded on the surface of the optical disc D.
The beam reflected from the surface of the optical disc D passes through the objective lens 4 and the collimator lens 3, and is reflected by the beam splitting prism 5 and detected by the optical detector 7 through the sensor lens 6.
In the conventional optical pickup apparatus, however, when a high density optical disc such as a Digital Video Disk (DVD) is used, the recording density is increased 6-8 times compared to using the optical disc of a conventional CD type. In order to reproduce data recorded on the high density discs, the numerical aperture of the objective lens 4 must be about 0.6. Therefore, the conventional optical pickup apparatus is inadequate to record and reproduce data on and from such discs.
In addition, the conventional optical pickup apparatus has the following problems which occur when reproducing data recorded on a high density optical disc, such as a DVD having a thickness of 0.6 mm, and an optical disc, such as a CD having a thickness of 1.2 mm.
FIG. 2 shows in a solid line the beam strength distribution when the beam is focused on the surface of the optical disc D having a thickness of 0.6 mm, using the objective lens 4 with a numerical aperture NA of 0.6. The objective lens is designed so that the beam can be focused on the surface of an optical disc D having the thickness of 0.6 mm.
In the beam strength distribution when focusing the beam on the optical disc D having a thickness of 1.2 mm by using the objective lens 4, the beam strength ratio of a main lobe is significantly reduced compared to the DVD having a thickness of 0.6 mm, and the beam strength of a side lobe is relatively increased, so that a crosstalk of the signal recorded on a neighboring track is increased.
For example, when reading data recorded on the optical disc (the conventional CD type) having a thickness of 1.2 mm and a track pitch of 1.6 .mu.m by using the objective lens 4 having a numerical aperture of 0.6, the crosstalk is increased by more than -20 dB.
Finally, when using the objective lens 4 having a numerical aperture (NA) of 0.6 in order to focus the beam on the surface of the optical disc D having a thickness of 0.6 mm, the spherical aberration with respect to the optical disc is significantly increased compared to the case where the optical disc having a thickness of 1.2 mm is used.
Therefore, it is impossible to stably reproduce the data recorded on the optical disc (the CD type) having a thickness of the 1.2 mm and on the DVD having a thickness of 0.6 mm by using the same conventional optical pickup apparatus.